Figure 4. Interactions of photosynthetic electron carriers with redox-signalling components of photosystem stoichiometry adjustment and state transitions.

Light reactions of photosynthesis are represented as electron transport from H₂O to NADP⁺ via two photosystems connected by a cytochrome b ₆ f complex which oxidizes plastiquinol (PQH₂) to plastoquinone (PQ). CSK senses the redox state of the plastoquinone pool directly by becoming autophosphorylated and activated by PQ. CSK phosphorylation and dephosphorylation initiate transcription of PS II reaction centre (psbA,D) and PS I reaction centre (psaA,B) genes, respectively, selectively controlling expression of reaction centre genes in chloroplast DNA. The LHC II kinase Stn7 responds to PQH₂ and initiates the state 2 transition, while the phospho-LHC II phosphatase, TAP38/PPH1, is redox-independent and predominates, inducing the state 1 transition, when PQ is oxidized. Even though they are both controlled by plastoquinone redox state, CSK exerts its transcriptional effect on photosystem stoichiometry independently of the effect of Stn7 in state transitions.